Magnetic resonance tomographs are imaging devices that, in order to represent an examination object, align nuclear spins of the examination object with a strong external magnetic field. The tomograph excites an alternating magnetic field for precession around the alignment. The precession or return of the spins from the excited state to a state with lower energy generates an alternating magnetic field. The alternating magnetic field is received via antennas.
With the aid of magnetic gradient fields, a spatial encoding is imprinted on the signals. The spatial encoding subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated and a three-dimensional imaging view of the examination object is provided.
The alternating magnetic field for excitation is dependent in its frequency on the static field used for alignment. The characteristic frequency is referred to as the Larmor frequency and has a value of 42.58 MHz/Tesla. As the intensity of the magnetic field increases, the frequency of the alternating fields therefore likewise increases.
Also associated with the higher frequency is an increased absorption in human tissue referred to as the SAR (specific absorption rate). For wearers of implants such as cardiac pacemakers, compliance with threshold values is of vital importance. For this reason, the standard IEC60601-2-33 provides a separate safe mode of operation for magnetic resonance tomographs, referred to as the “Fixed Parameter Option” (FPO). The FPO prescribes a limitation of operating parameters for the gradient and excitation fields of the magnetic resonance tomograph for the safe mode of operation.
In the safe mode of operation only head coils are permissible as local transmit coils. However, present day SAR-monitoring devices are not equipped to monitor the highly restricted threshold values for the FPO mode in conjunction with local coils.
The problem therefore arises of making the operation of a magnetic resonance tomograph safer.